Electrical steel sheets are generally classified into grain-oriented electrical steel sheet and non-oriented electrical steel sheet. Grain-oriented electrical steel sheet contains 3% silicon (Si) and has a texture in which grains are oriented in an orientation {(110)[001]}. Superior magnetic properties in the rolling direction allow these grain-oriented electrical steel sheet products to be used as core material of transformers, motors, generators and other electronic devices. Non-oriented electrical steel sheet is characterized, by orientations of grains being irregularly arranged and magnetic deviation according to magnetization direction being small. Due to these characteristics, the non-oriented electrical steel sheet is mainly used in a core for rotating machine such as generators or motors, in which magnetic flux direction is varied.
Recently, as electrical devices are diversified, demands on devices operating in high frequency band-increase and thus desires on core material with superior magnetic properties in high frequency also start to increase.
In the meanwhile, in alloys of Fe—Si, since higher silicon contents allow hysteresis loss, magnetostriction, coercive force, and magnetic anisotropy among core loss properties to decrease and maximum permeability to increase, it is said that high silicon steel products are superior soft magnetic material. Then, the decrease of magnetostriction and the increase of maximum permeability do not continue limitlessly according to the increase of silicon content but show maximum values in 6.5% Si steel. Also, it is well known that magnetic properties of 6.5% Si steel reach the maximum state in high frequency band as well as commercial frequency band. Due to the superior magnetic properties in high frequency band, high silicon steel is mainly applicable to high frequency reactor for gas turbine generator, tank power supply, induction heating device, uninterruptible power supply, or the like, and high frequency transformer for plating power supply, welding machine, X-ray power supply or the like, and is being used as substitution material. In addition, the high silicon steel is applicable for use to reduce power consumption of a motor and improve the efficiency of the motor.
Then, since elongation of the silicon steel sheet decreases abruptly as silicon content in Fe—Si steel increases, it is known that it is nearly impossible to manufacture the silicon steel sheet containing in excess of 3.5% Si by a cold rolling. In spite of such a fact that higher Si contents are effective in obtaining superior magnetic properties, the manufacture of such a high silicon steel sheet is recognized as a limitation of the cold rolling. Accordingly, researches on a new substitution technology that can overcome the limitation of the cold rolling are being tried from a long time ago.
Among the prior art methods for the manufacture of high silicon steel sheets, Japanese Patent Laid Open Publication No. 56-3625, discloses direct casting of high silicon steel using a single roll or twin rolls, Japanese Patent Laid Open Publication No. 62-10332 discloses a warm rolling in which rolling is performed in a heated state at a proper temperature, and Japanese Patent Laid Open Publication No. 5-171281 discloses a clad rolling in which rolling is performed in a state wherein the high silicon steel is located at an inner portion and a low silicon steel is located at an outer portion. However, the aforementioned prior art has not yet been commercialized.
For mass production of high silicon steel products such as 3% Si non-oriented steel products, a well known process includes the steps of depositing silicon on a surface of a material by a chemical vapor deposition using SiCl4 and then homogenizing the silicon, as disclosed in Japanese Patent Laid Open Publication No. 62-227078, U.S. Pat. No. 3,423,253, among others. However, the above process causes the manufactured products to be sold inevitably at a price five times higher than the conventional 3% Si steel products due to the difficulty in the CVD process. In spite of this fact that these products possess superior magnetic properties, it is difficult to popularize and commercialize such products due to the excessive high cost thereof.
Also, EP1052043A2, JP2000192204, JP2000144248, JP200045025, among others, disclose processes for manufacturing high silicon steel sheets using powder metallurgy. However, these prior arts have a limitation in that the high silicon content prevents the manufacture of steel sheet with the desired thickness.
Further, U.S. Pat. Nos. 3,634,148, 4,073,668 and the like propose a long-term annealing process in which Fe—Si alloy powder only or a mixed powder of Fe—Si powder and binder is prepared. The mixed powder is rolled at a reduction ratio less than 5% and then annealed for a long term. However, the process to coat powder on matrix material and then apply a rolling process makes it difficult to perform cold rolling and is also not desirable in a mass production system. Also, a low temperature long term annealing is not proper in mass production upon considering the productivity.
Among the currently circulated electrical steel products, only non-oriented electrical steel sheets containing 6.5% Si are produced and sold as the high silicon steel product. Owing to an irregular arrangement of grain, the non-oriented electrical steel sheets containing 6.5% Si content is used in the rotator with a small magnetic deviation according to magnetizing direction orientations. However, high silicon grain-oriented electrical steel sheet products, which demonstrate excellent characteristics in use for the transformer mainly using only the magnetic property in the rolling direction, have not yet been commercialized. Accordingly, various attempts to produce a grain-oriented electrical steel sheet with superior magnetic properties due to high silicon content have been performed, but it has not been informed yet on the success to produce such products.